My college son's 2018 2.0t (unmodified) had a dead battery the other day. Hasn't been driving it much lately and it is several years old, so no panic. Pulled battery, put it on a trickle charger over night. Battery tester indicates battery spanked. Was only putting out 8.4V and ~100 CCA. Go get new battery and installed same.

Disclaimer: I'm a very experienced mechanic and I know what I'm doing (Ford Mustang enthusiast, Jeep Wrangler & more). Anyway, I dropped both a nut and 10 mm socket below the battery tray. Wow, did that turn into a PITA trying to fish those out. I did use a magnet in this process but tried to stay away from the ECM/PCM which is obviously right there.

Get battery installed, NO I DID NOT INSTALL IT BACKWARDS BECAUSE YOU CANT. 12.4V, NO CRANK NO START. Christmas tree on the dash with all kinds of error messages. ABS system fault, transmission module fault, LKAS fault, TPMS system fault and more. Headlights, door locks, and key fob still work fine however.

Scan tool reads the dreaded U0100A & B codes - lost comms with ECM/PCM A & B (I think that is CANBUS high and low). I noticed on my OBDII connector under the dash there are very small scorch marks around pins 4-5-6 and 16. That's not good. Front probed pins 4 & 16 - the universal pins for power supply and I yet I was reading 12.4V.

Checked every single fuse under the hood and NONE are blown.

Carefully inspected the PCM/ECM and checked both large harness connections. Zero green crusties, no bent pins, excellent fitment.

Any thoughts and suggestions are appreciated! Thanks!

 

Did you check the fuses under the dash on the driver's side? I'm not sure if the 2018 has a fuse box under the passenger dash.

Not sure how you are checking fuses. I'd check each fuse while plugged in and use the test light method. That way, you can see if you are missing power feeding certain fuses and hopefully identify an electrical issue.

Does your scanner/code reader have any reset/relearn function for Hi/Lo CAN or battery replacement? I'm not familiar with the 2018 requirements, but remember reading some vehicles (BMW?) need a scanner when changing brake pads.

 

I have power going TO the underhood fuse box (aka relay control panel).

But NONE of the fuses - which should be constant power supply - have power to them. I checked them with a GE digital voltmeter and then also gently removed each one and visually inspected.

 

If none if the fuses are getting power and you have 12V to the under hood fuse box, start testing at the large 100A fuse and 50A fuse on each side. I'm not sure if you can test each side of the fuse when installed on a 2018. Power is going to be missing somewhere along the path pretty early with no fuses having power.